Tortured Plywood

The name 'tortured ply' certainly carries a negative ring. But within the limits of the system, that need not be so. The 'torture' comes from the fact that plywood is being bent more than is normal on a small boat and this is more safely achieved with the help of hot water applied to the outer veneers. Because plywood bends more readily at 90 degrees to the grain, it is used in that manner and for each half, the bend at the middle of the boat length will closely resemble a quarter-arc of a circle. So if two flat sheets are first attached together along their long edge, then the combined panel can be curved up to formed a shape close to that of a semicircle.

Claims that this 'torturing' will permit compound curvature are not strictly correct as only the very exterior wood fibres are affected by the damping—but still, such forming can certainly be more extreme that what one commonly experiences with dry plywood.

This semi-circular form lends itself admirably to long slim catamaran hulls, that do not depend on their individual shape for stability. (So by implication, it is therefore NOT suitable for monohull kayaks or canoes that need a tighter, more outboard bilge curve combined with some flat-of-bottom.)

The beam of any catamaran hull intended to be built using tortured ply, will depend on the thickness and type of plywood to be used. One of the most flexible of the marine plys is Okoume or Gaboon, so we will first consider working with that.

So just how far can you bend ply before it fails? Woods vary so much that it's hard to predict exactly, but let's have a shot at it. If we are talking about the recommended okoume marine ply, then from tests, I've found it can be bent in its easiest way (perpendicular to the grain) to about 300 × t², (with t in inches) and about 500 × t² if bent parallel to the grain.

By soaking the outer skins, I think it would be possible to use these figures for practical use for a tortured ply boat and if we also assume that we're to have a semi-circular section, then the minimum possible beam would equal 600 × t².

So based on the above formula, here is a rough graph of what 'minimum beam' you probably need to accept for a semicircular hull, plotted against each specific thickness of plywood. Each sheet will no doubt vary slightly from these test figures and plys of ⅜" and thicker are likely to be proportionally stiffer as the wetted surfaces will affect a lower percentage of the total plywood thickness, but the graph can still be a good starting point.

Tests I've conducted on good samples, have shown that one can even go 10–15% under these values, but it's chancy. Even to get these figures, one needs to proceed slowly and symmetrically to avoid material failure, but they are doable.

However, if you are using denser plywood (such as a meranti or equal), you'd have to increase these figures by about 50% and with Douglas Fir plywood, I am not sure I'd even try it as it splinters too easily and is typically full of surface flaws.

This explains why the most common use for tortured ply is with very thin plywood—seldom over 3⁄16" and more frequently even ⅛". (However, one could always consider using more than one layer and vacuum-bag two skins together with epoxy after their wet surfaces had totally dried.)

The most famous use of tortured ply is probably its original use to construct hulls for the lightweight 'Tornado' catamaran. In 1979, using 4.5mm ply for the hulls, she outperformed all the other B-class competition, to then be selected as a new Olympic Class - still raced today. Although the method is still used by some today, the actual Olympic boats are now honeycomb sandwiched in graphite. The tortured ply system is more difficult to build to exact dimensions. Here is a picture from the original building manual. More details on how a Tornado hull can be created, can be found online at: www.instructables.com

You'll note that the interior is lightly glassed and a foam/glass stiffener is incorporated into the forward ½ of the hull about ½ way down the hulls. The picture (above left) is from Gougeon's fine boatbuilding book.

The process is to first attached the two side sheets together and then, after soaking the part of the ply requiring the most curve, to curve up the sides with the help of ratchet straps until the deck edge can be fitted into a pre-built deck jig. This will considerably help to control the final shape and once the gunwales and any bulkheads are fitted, the hull is reasonably stable in shape. Once totally dry, the thin skin can be epoxied and even lightly sheathed for additional stability and strength.

Some claims have been made that tortured ply hulls have poor shape and 'plunge through waves' etc. but this depends on what they are used for and the Tornado example is a proof that some can work very well. The two ends can be very differently formed and that helps considerably to control pitching.

Photos of a Taipan catamaran built with tortured ply. Build & photo credits go to Phill Brander, with thanks.

As far as using the system for larger boats, it's seldom been done. One could consider using two layers of thin ply to get a better shape through smaller radii or the chart above indicates that a 5-ft beam hull could be made with 5⁄16" ply and if heavily sheathed for strength with FG cloth and added stringers, this could conceivably work for a trimaran main hull—though in such a case, it would be a design more for space than for speed. Depth could be added with an additional strip of ply joined with a 3–4" butt-strap located under a stringer.

Additional fine bow form can be obtained by cutting slits in the forefoot of a plywood hull and then backing this area with another plywood on the inside. Although not a tortured ply design, the bow of the neat Cherub dinghy from Australia is formed this way. See drawing at right.

Advantages:

The system is simple, inexpensive and quick.

The only building frame or jig is really the deck frame.

Disadvantages:

Boat section shapes are very limited. The ply will bend to create close to a ½ circle bilge at the stern.

The ply panel will be under some stress for quite a long time and this can make it more susceptible to damage if further stressed or impacted.

To help deal with the stress, examples of this concept beyond that of a beach cat, will really require fibreglass sheathing and this will add weight and cost.

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